This invention relates to a monitoring device operatively connectable to an electrical device, such as an LC filter or a capacitor battery, for evaluation of internal faults therein causing changes in impedance. The electrical device is connected to an electric power supply network via a switch which may be opened by the monitoring device upon the detection of a fault of a predetermined nature.
German Patent Document (Offenlegungsschrift) No. 30 20 128 discloses a monitoring device for detecting faults in a capacitor battery comprising a multiplicity of parallel capacitor circuits connected in series to one another. Each capacitor circuit comprises a multiplicity of parallel circuit legs each including a capacitor component connected in series to a respective fuse. The capacitor battery is connected to a power supply network.
Upon the short circuit of an individual capacitor in a capacitor battery, the associated fuse reacts to form an open circuit. The monitoring device of Offenlegungsschrift No. 30 20 128 detects such internal defects without differentation and indicates their occurrence separately in accordance with their relative positions in time. The monitoring device also indicates to an operator the total number of defects. Thus, for example, a first defect is indicated in the same manner and by the same indicating device, regardless of the nature of the defect. A second defect is similarly signaled by a second indicating device with no specification as to the type of fault. It is frequently left to the judgment of operating personnel whether the electrical device being monitored is to be disconnected from the power supply network. As set forth in Offenlegungsschrift No. 30 20 128, the electrical device may be automatically disconnected upon the occurrence of two or more faults.
The monitoring device disclosed in Offenlegungsschrift No. 30 20 128 comprises current and voltage transformers operatively connected to the capacitor battery for sensing the voltage drop thereacross and the current flow therethrough. An electronic simulation circuit is connected to the current transformer for generating an estimated value of the voltage drop across the capacitor battery, whereby the so-called "observer principle" may be applied. The electronic simulation circuit is based upon the equivalent circuit of the capacitor battery. The simulation circuit is designed to compensate for slow changes in the electrical operating characteristics of the capacitor battery, such changes arising from aging influences, temperature influences and other normal external influences.
In order to detect abrupt changes in the operating state of the capacitor battery due to internal faults thereof, the fundamental frequency of the actual voltage drop across the capacitor battery and the fundamental frequency of the estimated voltage drop across the capacitor battery are extracted by filters and are subsequently converted into amplitude-proportional d-c voltages. These d-c voltages are transmitted to a transient comparator circuit operating as a nonlinear control device. The transient comparator circuit detects differences between the rectified fundamental frequencies of the actual and the estimated voltage drop across the compacitor battery, the differences resulting from internal faults in the battery. The differences detected by the transient comparator circuit are evaluated at the output thereof, the evaluation being directed only to number of faults and not to their types. Indication of the first, second and other faults is accomplished via separate indicating devices, while the capacitor battery can be disconnected from the power supply network automatically if two or more faults occur.
The fact that the manual or automatic disconnection of the capacitor battery from the power supply network may be accomplished on the basis of the number of faults without regard to the type of faults can lead either to premature disconnection in the case of essentially harmless defects or to late disconnection in the case that a singly occurring fault is serious.
The monitoring device disclosed in Offenlegungsschrift No. 30 20 128 also works in cases where the changes in input voltage are large and fast. However, the monitoring device cannot be used to protect other kinds of electrical devices such as LC filter circuits and electrical machines.
An object of the present invention is to provide an improved monitoring device of the above-described type for protecting electrical devices which have inductive and/or ohmic components, as well as capacitive components.
Another, more particular object of the present invention is to provide such a monitoring device in which not only the number of faults, but also the type of faults can be determined, whereby selective measures may be undertaken for automatically tripping a network switch to disconnect the protected device from a power supply network.